Gas turbine engines typically include a compressor section, a combustion section and a turbine section. The compressor section pressurizes ambient air that discharges at high pressure into a combustion section, typically comprising a plurality of combustors positioned in an annular array about the axis of the gas turbine engine. The pressurized air flows into the combustor fuel nozzles through one or more openings in the nozzles and mixes with a fuel source. The fuel/air mixture is injected into and burned in the combustion chamber of each combustor and the hot gasses of combustion flow from the combustion section to the turbine section where energy is extracted from the gasses to drive the turbine and generate electric power.
In certain gas turbine applications, such as oxy-fuel or stoichiometric exhaust gas recirculation (“SEGR”) systems, the compressor discharge typically includes an oxygen-deficient gas because such systems, particularly SEGR, are designed to minimize the amount of excess oxygen present in order to maintain an acceptable level of combustion efficiency. Normally, any excess oxygen in SEGR systems eventually must be removed from the working fluid in order to ensure optimum engine performance and efficiency.
One area of concern with SEGR systems is that the low levels of oxygen present at certain locations within the SEGR loop can cause the metal oxide coatings used on critical engine components to deteriorate over time by reverting to a non-oxidized form of the coatings. Such metal oxide coatings normally fall into three basic categories: (1) corrosion-resistant coatings; (2) oxidation-resistant coatings; and (3) thermal barrier coatings (“TBC”), as well as combinations thereof. The deterioration of the oxide coatings over time can result in a potential loss of their protective and structural attributes and damage to one or more key component parts.
In recent years, the potential for oxide degeneration of components in gas turbine engines, particularly SEGR systems, has become greater due to the higher operating temperatures used to improve engine efficiency over long periods of operation. As the operating temperatures increase, the durability of the components of the engine at elevated temperatures must correspondingly increase. While significant advances in metallurgy in high temperature applications have been made using, for example, nickel and cobalt-base superalloys, the use of alloys alone often is not adequate to protect turbine components in certain critical operating sections of a gas turbine engine that are vulnerable to oxide degradation over time. One past solution has been to thermally insulate critical components (e.g., the turbine blades) to reduce the effective service temperature of the metal substrate. For example, thermal barrier coatings (“TBC”) can be applied over the metal substrate of certain components to improve their long term reliability and typically comprise a metal oxide layer placed over the base metal substrate using a bond coat applied by known thermal spray techniques such as physical vapor deposition.
Although significant advances have been made in improving the durability of TBC, oxidation-resistant and corrosion-resistant coatings, such techniques invariably require removal and repair of the deteriorated metal components after extended periods of use. The physical removal of protective coatings and the repair of underlying metal substrates can be very time consuming and even result in a loss of the underlying metal substrate in order to undertake the repairs. The potential removal of the underlying metal substrate is particularly acute with diffusion coatings and bond coat layers since the coatings/layers often extend over time into the metal substrate surface. As noted above, repeated conventional repair/recoat processes can result in significant material losses that eventually cause the component to be under minimum tolerable wall thicknesses and thereby vulnerable to catastrophic failure.
Various methods have also been used in the past in an effort to protect the underlying base metal components in new gas turbines. Significantly, however, the known methods invariably require that the protective coatings be applied to specific components before the engine is placed into operation (or perhaps later during a shutdown repair mode). Thus, such methods cannot be used to accomplish an in situ repair while the system is up and running. In the past, various protective coatings have been applied to select engine components using conventional plasma spray techniques before the engine becomes operable. Other processes improve the surface prophylaxis of specific components against fouling by contacting the surfaces with a metal compound that converts into a metal oxide during a subsequent heat treatment.
Again, these known methods do not teach or suggest a technique for regenerating the metal oxide coatings in situ while the engine remains in full operation, particularly gas turbine engines that utilize SEGR and run with low oxygen concentrations in the working fluid in order to achieve acceptable operating efficiencies. For example, a level of 1% oxygen by volume is normally considered the maximum level that most SEGR systems can tolerate without sacrificing efficiency. In the past, the prospect of injecting additional air or oxygen into an SEGR loop to improve the integrity or projected life of the underlying oxide layers was considered detrimental to the overall engine operation and counter-intuitive because of the predicted engine efficiency losses. Although oxygen levels at 1% or lower in an SEGR loop help to maintain desired engine efficiency levels, over long periods of time the low level of oxygen tends to cause the oxide coatings of key SEGR components to become reduction targets resulting in an eventual loss of metal integrity and strength. Eventually, the entire engine must be taken out of service in order to repair and/or refurbish those components.
Thus, a significant need still exists in the art for an improved process to repair gas engine turbine components during operation, particularly those involving SEGR, in order to minimize the loss of the underlying metal substrate and/or to regenerate the base oxide coatings. A need also exists to preserve the existing metal oxide coatings and substrates, particularly in SEGR systems, without undertaking costly repairs and the inevitable downtime of refurbishing selected metal components which require taking the entire gas turbine engine out of service for an extended period of time. The oxidation reactions regenerating the oxide coatings is more rapid at the higher metal temperatures during operation reducing the regeneration time.
The general categories of articles that can be treated as described herein include those comprising a base metal substrate having a first material containing one or more metal oxides and a layer of second material overlying at least a portion of the metal substrate. The second material normally will be similar in composition to the first material with the base substrate and layer being integrally bonded at their interface. Often, the second layer to be regenerated comprises material from a deposition process such as vapor phase deposition, ion plasma deposition, cathodic arc deposition sputtering techniques or combinations thereof.